[#absl-call_once]
= xref:absl.adoc[absl]::call_once
:relfileprefix: ../
:mrdocs:

For all invocations using a given `once_flag`, invokes a given `fn` exactly once across all threads. The first call to `call_once()` with a particular `once_flag` argument (that does not throw an exception) will run the specified function with the provided `args`; other calls with the same `once_flag` argument will not run the function, but will wait for the provided function to finish running (if it is still running).

== Synopsis

Declared in `<absl/base/call_once.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    typename Callable,
    typename... Args>
void
call_once(
    xref:absl/once_flag.adoc[absl::once_flag]& flag,
    Callable&& fn,
    Args&&... args);
----

== Description

This mechanism provides a safe, simple, and fast mechanism for one‐time initialization in a multi‐threaded process.

Example:

class MyInitClass { public: ... mutable absl::once_flag once_;

MyInitClass* init() const { absl::call_once(once_, &MyInitClass::Init, this); return ptr_; }

== Parameters

[cols="1,4"]
|===
| Name| Description
| *flag*
| The `once_flag` used to distinguish this call from others.
| *fn*
| The callable to invoke at most once.
| *args*
| The arguments to pass to `fn`.
|===


[.small]#Created with https://www.mrdocs.com[MrDocs]#